1. Field of the Invention
This invention generally relates to semiconductor fabrication, including polycrystalline silicon thin film transistors (TFTs) for active matrix (AM) liquid crystal displays (LCDs) and, more particularly, to a system and method for using a symmetrically patterned mask in laser annealing operations.
2. Description of the Related Art
FIG. 1 is an illustration of a laser annealing apparatus (prior art). One method to crystallize amorphous silicon (a-Si) films and obtain polycrystalline silicon (poly-Si) is by laser annealing. In one implementation of laser annealing, the laser beam is xe2x80x9cshapedxe2x80x9d by a mask that is inserted into the path of the beam. The shaped laser beam is then projected into the surface of the film to be annealed, by passing through an appropriate projection lens and associated optics.
The mask is kept at the mask stage. The substrate steps under the laser beam by means of a stage that can very accurately position and move both the mask and the substrate under the pulsed laser beam. The substrate and mask move in a serpentine pattern. The mask that is used to shape the laser beam can have a single pattern, or a variety of patterns on it. In the latter case, each time the laser discharges, the substrate and mask move at a precise distance, bringing the same area of the substrate under a different pattern of the mask. This allows the same area of the substrate to be irradiated under different conditions, with the objective of improving the resulting microstructure in a sequential manner. For example, one pattern of the mask may generate large lateral grain growth, another pattern reduces the density of grain defects generated after exposure to the first pattern, and a third pattern xe2x80x9csmoothes-outxe2x80x9d the surface of the poly-Si structure generated after exposure to the 1st and 2nd patterns. Generally, there may be as many patterns on the mask as the task requires. However, even though the number of patterns is unlimited, the exposure has to follow a specific sequence to have the desirable effect.
FIG. 2 illustrates a plan view of a mask with three patterns (prior art). Substrate motion under the mask is considered to be unilateral if the motion in one direction is followed by a motion in the opposite direction, after an edge of the substrate is reached. Although this movement is the most cost effective manner of operating the apparatus, a conventional multi-pattern mask cannot be used with such a unilateral motion. The three patterns create a specific exposure sequence in one direction that reverses when the substrate moves in an opposite direction. If the three-pattern sequence generates a beneficial effect in one direction, the effect is destroyed when the substrate motion is reversed to complete the serpentine pattern of exposure. Therefore, multi-pattern masks are conventionally used with respect to a single substrate direction. The time it takes for the substrate to reverse directions for the next pass of multi-patterned laser shots is wasted process time.
It would be advantageous if the laser annealing process time associated with the use of a multi-pattern mask could be improved.
It would be advantageous if a multi-pattern mask could be used in a laser apparatus that moves the substrates with a unilateral motion.
The present invention is invention discloses two related processes that enable a multi-region, or multi-pattern beam-masking scheme to be used with unilateral motion of a substrate. One process uses a pair of reciprocal pattern masks. One mask is used for substrate motion in a first direction, while the other mask is used for substrate motion in the opposite direction. The second process uses a single mask with reciprocal patterns.
Accordingly, a method is provided for efficiently laser irradiating a semiconductor substrate. The method comprises: exposing a semiconductor substrate to laser light projected through a multi-pattern mask; advancing the mask and substrate in a first direction to sequentially expose adjacent areas of the substrate to each of the mask patterns in a first predetermined order; and, advancing the mask and the substrate in a second direction, opposite the first direction, to sequentially expose adjacent areas of the substrate to each of the mask patterns in the first order.
In one aspect, the method further comprises: forming a multi-pattern mask having a first plurality patterns aligned in the first order with respect to the first direction and a second plurality of patterns, corresponding to the first plurality of patterns, aligned in the first order with respect to the second direction. Alternately, the method comprises: forming a first multi-pattern mask having a first plurality patterns aligned in the first order with respect to the first direction; and, forming a second multi-pattern mask having a second plurality of patterns, corresponding to the first plurality of patterns, aligned in the first order with respect to the second direction. Then, advancing the mask and substrate in the first direction includes using the first mask, and advancing the mask in the second direction includes using the second mask.
Advancing the mask and substrate in a first direction to sequentially expose adjacent areas of the substrate to each of the mask patterns in a first order includes exposing a first area of the substrate to laser light projected through a mask Region 2, followed by laser light projected through a mask Region 1. Likewise, advancing the mask and substrate in the second direction, opposite the first direction, to sequentially expose adjacent areas of the substrate to each of the mask patterns in the first order includes exposing a second area of the substrate to laser light projected through the mask Region 2, followed by laser light projected through the mask Region 1. The method further comprises: forming a first condition in the substrate in response to light projected through mask Region 2; and, modifying the first condition in the substrate in response to light projected through mask Region 1.
Additional details of the above-described method, and a system for efficiently laser irradiating a semiconductor substrate using a multi-patterned mask are presented below.